LCD

TFT

LED-OLED

CCD

CMOS

DISPLAY SYSTEMS AND PHOTOSENSORS (PART 1)

LCD Display

M.A.MOEENI

SIMPLE TYPES OF LC

-Three major characteristics of Liquid Crystal

- The Thermal Nature

High Temp

Clearing PointMelting Point

Liquid StateLiquid Crystaline StateSolid State

(Crystal)

- The Optical Nature of a LC molecule

LightNO change in polarization state

Phase retardation will exist

Low Temp

- The Electrical Nature of the LC molecules

AC potentialNo potential field

Electrodes

PROPERTY OF LIQUID CRYSTALS (LC)

THE HISTORY OF LIQUID CRYSTAL DISPLAY LCD

Before the appearance of LCD, it is the Cathode Ray Tube (CRT) kingdom. The CRT monitors dominate almost all the display applications. The CRT, invented by Karl Ferdinand Braun, is also called Braun tube.

Cathode rays exist in the form of streams of high speed electrons emitted from the heating of cathode inside a vacuum tube at its rear end. The released electrons form a beam within the tube due to the voltage difference applied across the two electrodes, and the direction of this beam is then altered either by a magnetic or electric field to trace over the inside surface of the phosphorescent screen (anode), covered by phosphorescent material.

Light is emitted by that material at the instant that electrons hit it.

- good image quality- no problem with response time - no problem with viewing angle But it is bulky and high power consumption, So Display engineers tried looking for alternative technologies like flat panel display.

In 1990s, technology breakthrough brought the birth of active matrix LCD, along with the plasma display, both of which become the main stream of the flat display markets, replacing the CRT

LCD DISPLAY

(A) It has a mirror which makes it reflective

(B) & (F) piece of glass with a polarizing film on the bottom side

(C) & (E) a common electrode plane made of indium-tin oxide on top

(D) layer of liquid crystal substance

THREE COMMON TYPES OF LCD

Light(Back Light)

- TRANSMISSIVE TYPE

- REFLECTIVE TYPE

POLARIZER ON THE FRONT SIDE REFLECTOR ON THE BACK SIDE

Incident Light

- TRANSFLECTIVE TYPE

Day LightNight Light(Back Light)

TRANSFLECTOR ON THE BACK SIDE

POLARIZER ON BOTH SIDES

POLARIZER ON THE FRONT SIDE

LCD

LCD

LCD

Eyes

An alternative way to achieve high-resolution LCD is to use the Liquid Crystal on Silicon (LCOS) devices. LCOS devices use only one glass substrate, and employ a silicon wafer for the back substrate. The pixels are then generally coated with a reflective aluminum layer, and then a polyimide alignment layer.This technology can also be used in personal viewer such as viewfinder in digital cameras and camcorders.

LCD MODES

Along with the development of LCD's driving infrastructure, different LCD modes were introduced to improve the image quality.

-ASV-BINEM-Cholesteric-ECB-SSFLC-Guest-Host-IPS-LCOS-MVA-PDLC-Pi-Cell-PVA-STN-TN

George Heilmeier in 1968

TWISTED NEMATIC (TN) MODE

The TN mode is the "workhorse" for the LC display. It was first introduced by Schadt and Helfrich, and also by Fergason in 1971

TWISTED NEMATIC (TN) MODE

The gray scale is achieved by applying intermediate voltages between 0 and the value at which light is completely blocked.

A TYPICAL TN TYPE LCD CELL

With AC Voltsconnected

NO powersupply

Polarizer(Axis 90 degrees)

Light

Cell Gap The separation between two glasses

Depending on how the LCD fluid is formulated.

Glass with electrodes

Polarizer(Axis 0 degree)

The smaller the cell gap, the faster response.

TWISTED NEMATIC (TN) MODE

TWISTED NEMATIC (TN) MODE

For a LCD, each pixel is divided into three subpixels, which have red, green and blue color filters.

The exact color coordinates of the white point depend on the relative transmission and color purity of the red green and blue subpixels.

ADVANCED SUPER VIEW (ASV) MODE

The ASV mode was developed by Sharp.

Because of the full circle rotation of the director, the viewing cone is very symmetric and viewing angle performance is excellent.

POLYMER DISPERSED LIQUID CRYSTAL (PDLC) MODE

-The PDLC display consists of droplets of liquid crystals inside a polymer network

In the off state, the droplets are randomly aligned hence the light is scattered in a large angle towards the viewer.

In the on state, light can be transmitted with a very high transmission.

POLYMER DISPERSED LIQUID CRYSTAL (PDLC) MODE

The working voltage and response time of the PDLC can be affected by:

- the resistive and dielectric properties of LC inside the droplet and the polymer properties- size of the droplets - shape of the droplets- the viscosity of the of the droplets

There are a few factors influencing the contrast ratio of the PDLC display:

- the cell gap - the density of the droplets

GUEST - HOST (GH) MODE

In a Guest - Host system, the mixture is prepared by mixing LC and dichroic dyes.

The dichroic dyes absorb the light whose E-field is along the long axis of the dye.

When the LC molecules change their orientation, the dye will also change along with LC molecules, consequently, the absorption axis is changing, a light transmission can be modulated.

There are three simple GH displays:

- The Heilmeier type GH- The Double Layer type GH - The PDLC type GH.